Osteoarthritis is the leading cause for joint replacement surgery worldwide. Although the bone may eventually be involved, osteoarthritis is primarily a disease of cartilage. Bones have sensory nerves just like skin. These nerves exist on the surfaces of the bone both on the femoral head and acetabulum. Normally the bone surfaces along with their sensory nerves are covered by articular cartilage or hyaline cartilage. Hyaline cartilage is unique in that not only does it not have a blood supply, it also does not possess a nerve supply, i.e., it is aneural. Therefore, as long as there is cartilage interposed between the joint surfaces, since there are no nerves, there is no pain. The pain of osteoarthritis is generated once the cartilage has eroded away, and there is resulting bone on bone contact or nerve on nerve contact.
There are three basic classifications of joints of the human body: synarthroidal, amphiarthroidal, and diarthroidal. Synarthroidal joints provide immovable articulations; amphiarthroidal joints provide mixed articulations; and diarthroidal joints provide movable articulations. Healthy fibro cartilage and hyaline cartilage within the joint provide a weight-bearing function and allow painless articulation of amphiarthroidal and diarthroidal joints.
Primary osteoarthritis is a debilitating disease that affects amphiarthroidal and diarthroidal joints. The changes that occur with primary osteoarthritis involve altered biomechanical, biochemical, histological and metabolic characteristics of the cartilage, synovial fluid and bone. Initially, these changes affect the articular cartilage and eventually affect the surrounding perichondral tissues in a cascade of events.
Articular cartilage, also called hyaline cartilage, is made of a multiphasic material with two major phases: a fluid phase composed of water (68%-85%) and electrolytes, and a solid phase composed of collagen fibrils (primarily type II collagen, 10%-20%), proteoglycans and other glycoproteins (5-10%), and chrondrocytes (cartilaginous cells). 30% of all cartilage water resides in this interstitial fluid, and this amount does not vary with age. However, there is a significant increase of total amount of water in degenerating cartilages. This multiphasic system allows fluid flowing from the tissue to the solution surrounding the tissue, and vice versa, through the pores of the collage-proteoglycan solid matrix. As the fluid passes to the pores, the force exerted on the walls of the pores causes more compaction. Thus, it becomes more and more difficult to squeeze fluid from the tissue with prolonged compression. This non-linear flow-induced compression effect is very important in the physiology of cartilage not just because it determines cartilage compressive behaviors, but also because it provides the mechanism for energy dissipation.
There are many theories concerning how articular cartilage functions as a weight bearing surface, which include hydrodynamic, boundary, elastohydrodynamic and squeeze film lubrication. However, it is known that the viscoelastic properties contribute to the multiple functions of articular cartilage, including its weight bearing function. The viscoelastic properties of cartilage are due to an intricate tight meshwork of interlacing collagen fibers that physically ensnare the large macromolecules of proteoglycan.
To date, treatment of osteoarthritis has been with the use of total joint replacement surgery. This entails resection of the proximal femur (femoral head and neck), reaming of the femoral intramedullary canal and the insertion of one or more modular artificial metal component(s) to replace the diseased cartilage on the resected bone. Similarly the acetabulum is removed by reaming the socket down to bleeding bone and the impacting of an artificial socket into the pelvis. The two components are then joined by suturing the dissected surrounding tissues together, joining the two components into contact with each other. The materials used for these devices are usually an alloy of various metals typically cobalt, chrome and titanium. The bearing surfaces vary from polyethylene on metal, metal on metal, ceramic on ceramic, and various combinations of them all. The operations are extensive dissections with implantation of large quantities of inert material into the human body. Potential complications are extensive and can range anywhere from minor wound complications to death of the patient. All approaches entail the complete replacement and substitution of the joint with artificial components with their own inherent mechanics of joint function. It would be desirable to provide a method and apparatus for treating osteoarthritis that minimizes surgical intervention and human tissue resection and substitution.